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Is childhood obesity contagious?

Every parent knows that young children catch lots of things at school: chicken pox, the flu and, of course, the annual back-to-school cold.

Now there’s evidence that kids can catch something else from their classmates: obesity.

Of course there isn’t a fat virus, or at least one we know about. But a research team from the University of Arkansas has tracked the pattern of weight gains and obesity among 341,876 elementary school students and found that the typical student gained extra pounds in grades with a larger share of obese classmates but slimmed down when they were in classes with a larger proportion of skinny kids.

Hispanic and black children were particularly susceptible to exposure to fat peers, they found. Even an increase in the share of children who were considered overweight but not obese had a corresponding effect on other students’ weight, Jebaraj Asirvatham reported in a paper the team will deliver next month at the Agricultural & Applied Economics Association meeting in Washington, DC.

The effect is striking. As the proportion of obese students in a grade doubles, “a typical normal-weight child nearly becomes overweight,” Asirvatham said in an email.

Asirvatham says their study shows that changes in a student’s weight are strongly correlated with the weight of their peers. But he cautions that more work is needed before researchers can claim a causal relationship between the two or pinpoint what the underlying mechanisms causing the change might be.

Their conclusions flow from an analysis of a unique database that contains the body mass index (BMI) scores of every Arkansas public elementary student from 2004 to 2010. These BMI scores, which measure body fat based on an individual’s height and weight, were collected every two years, allowing researchers to track individual children and their classmates over time.

To help ensure they measured the effects of the school environment, they controlled for the presence of fast food restaurants, convenience stores and other fatty food venues in the area surrounding the school and in the student’s home neighborhood. Overall, Arkansas grade school students rank in the 69th percentile among students nationally in terms of BMI scores.

Asirvatham and his colleagues discovered that as the share of overweight students in a class rose, so did the BMIs of other students. For example, they found that if the percentage of obese students doubled, the percentile ranking of the typical student’s BMI rose from 69 to 82—“only 3 percentile ranks below being classified as overweight,” he said.

The weight gain effect was particularly strong among Latino and black students. “An average Hispanic student at the 70th percentile moved to the 85th percentile or became overweight,” Asirvatham and his colleagues found. “The increase in body weight for an African American student was a little lower, i.e., he [or] she moved to the 84th percentile. The increase was lowest for a Caucasian student, who moved to the 79th percentile. Thus Hispanic and African American students seem to be more influenced by peers than Caucasians.”